The use of dielectric rods as waveguides for propagating electromagnetic waves in the microwave and millimeter wave region of the spectrum is well known: see "An Investigation of Dielectric Rod as Wave Guide", by C. H. Chandler, Journal of Applied Physics, Vol. 20 (December 1949), pages 1188-1192.
In the past waveguides of the foregoing type usually were constructed with materials having relatively low dielectric constants (e.g., polystyrene, quartz, and Teflon). In order for propagating millimeter waves to be confined within rods of low dielectric constant material, rod diameters are required which are excessively large for a number of applications. On the other hand, when such rods are only a fraction of a wavelength in diameter, the greater part of the propagating wave energy lies outside of the rod, creating evanescent fields which make it extremely difficult to support the rods in a practical manner. Moreover, when dielectric rods which propagate large evanescent fields are bent or have other surface imperfections, considerable power may be lost by radiation.
Alternatively, previous rod waveguide materials with high dielectric constants (e.g. gallium arsenide, silicon, and sapphire) were quite rigid, and waveguides which could be fabricated from these materials were limited to a few centimeters in length. Thus, flexible, long, readily supportable dielectric rod waveguides for millimeter waves were beyond the state of the art.
A further area of prior art of relevance to the present invention but which heretofore was never associated with millimeter wave propagation is that relating to optical waveguides using fibers of thallium bromo-iodide, alternatively known as KRS-5. Crystals of thallium bromo-iodide have long been used for the refraction and dispersion of light, particularly at infrared wavelengths, as discussed in a paper by William S. Rodney and Irving H. Malitson, "Refraction and Dispersion of Thallium Bromide Iodide", Journal of the Optical Society of America, Vol. 46, No. 11 (November 1956), pages 956-961. More recently, extrusion techniques have been devised for producing co-crystalized fibers of thallium bromo-iodide in continuous lengths of up to 200 meters. These extrusion techniques are described in detail in patent application Ser. No. 37,581, filed May 9, 1979 by Douglas A. Pinnow et al and entitled "Infrared Transmitting Fiber Optical Waveguides Extruded from Halides", which application is a continuation of application Serial No. 800,149, filed May 24, 1977, now abandoned and in a paper by D. A. Pinnow et al "Polycrystalline Fiber Optical Waveguides for Infrared Transmission", IEEE Journal of Quantum Electronics, QE-13, No. 9 (September 1977), page 91D.
Thallium bromo-iodide fibers made by the aforementioned extrusion techniques have been found to be optically transparent over a range of light wavelengths from approximately 0.6 .mu.m in the visible region to approximately 35 .mu.m in the infrared region, and hence are particularly suited for use as a fiber optical waveguide for the transmission of light at infrared wavelengths. However, prior to the present invention there was nothing to suggest that such fibers also could be used for propagating electromagnetic waves at millimeter wavelengths. In fact there is a dearth of published literature on appropriate parameter values (e.g. dielectric constant and loss tangent) which would give any clue to the usefulness of co-crystalized thallium bromo-idoide for millimeter wave propagation.
More specifically, in the book by A. R. Von Hippel, Dielectric Materials and Applications, Technology Press of MIT and John Wiley, New York (1954), page 302, values are given for the dielectric constant and loss tangent of thallium bromo-iodide for a number of radio frequencies ranging from 100 Hz to 10 MHz; however, no values are given corresponding to wavelengths shorter than 30,000 mm. The only other previous radio frequency measurements known for thallium bromo-iodide are the dielectric constant measurements of R. C. Powell of the National Bureau of Standards Boulder Laboratories at wavelengths of 300 mm and 1500 mm, and which are given on page 958 of the aforementioned Rodney and Malitson paper.